The invention relates to systems that are provided with batteries formed by battery modules arranged in series, and particularly to electric or hybrid powertrain automobiles.
These systems can comprise a load to be supplied with power, for example, in the case of a vehicle, an electric powertrain supplied with electricity by a battery. This battery is generally composed of a plurality of battery modules arranged in series, and the modules can be independently series connected or not, which corresponds to an activation or a deactivation. In other words, the number of battery modules activated varies depending on the voltage that one wishes to supply to the load.
A capacitor is arranged between the battery and the load so as to smooth out the high frequency currents but also the transient currents absorbed by a converter stage which can be an inverter arranged at the input of the powertrain. When the battery (that is to say the group of modules) is connected by relay to the powertrain, for example, during starting of the vehicle, high transient currents appear and can damage the components, for example, the capacitor, the relays used or the power switches.
Furthermore, a rapid voltage variation at the terminals of the inverter stage arranged at the input of the powertrain cannot be brought under control fast enough by the inverter stage, which can generate jolts that are felt by the driver of the rolling vehicle. These jolts can damage the mechanical elements of the powertrain.
In vehicles, pre-charging the capacitor just during starting of the vehicle has been proposed, using a circuit comprising two contactors and a resistor in series with the capacitor. A first contactor makes it possible to charge the capacitor through the resistor with a reduced current, and a second contactor makes it possible to by-pass the resistor for normal operation. This solution has the disadvantage of requiring a circuit comprising expensive and cumbersome contactors, and the contactors run the risk of remaining closed if they are exposed to excessively high currents.
One can refer to the European patent application EP 2 361 799 which describes a system for increasing the voltage at the terminals of the capacitor before connecting the powertrain, that is to say a system for pre-charging the capacitor. In particular, this document proposes using a resistor in order to form a R-C type circuit for charging the capacitor. This system has the disadvantage of not enabling one to obtain complete charging of the capacitor, the charging time taking too long. This system also has the disadvantage of adding a resistor in series to the system. This system therefore cannot be used during starting of the vehicle because of this resistor. It is therefore not suitable for activation or deactivation of a battery module during the operation of the vehicle.
One can also refer to the American patent application US 2012/0025768 which describes a system for pre-charging a capacitor having a low-value resistor, and a switch unit for improving the pre-charging of the capacitor. This system cannot be used during operation of the vehicle either.
The international patent application WO 2009/077668 describes a system for pre-charging a capacitor in which a switch is connected in series to the capacitor, and this switch is controlled with a pulse width modulation signal whose duty cycle varies. This solution has the disadvantage of increasing the impedance of the branch containing the capacitor.
Finally, one can refer to the French patent application FR 2 923 962 which describes another system for pre-charging a capacitor using a “boost” type power converter but which also has the disadvantage of increasing the impendence of the branch containing the capacitor. It is consequently impossible to use this system during operation of the vehicle because of the excessive losses due to the increase of the impedance.